Many optical fiber communication systems today transmit data over long distances with higher data rates or with a larger number of data channels. To help reduce the capital and operating costs associated with transmitting data, such systems are currently being built to transmit data, video and voice over these longer distances without employing optical regeneration or other forms of repeaters. The transmission capacity of a fiber depends on the modulation format utilized, transmission and reception equipment utilized, as well as the intrinsic properties of the fiber itself, including its length. Certain effects, such as chromatic dispersion, waveguide dispersion, Four-Wave Mixing (FWM), Raman Coupling, and Stimulated Brillouin Scattering increase as a strongly nonlinear function of the length of the fiber, which further tends to lower the achievable transmission rate. To transmit the greatest amount of information per fiber—specifically in the case of information which is frequency-division formatted so as to fit within discrete “channels”—the greatest number of channels in an optical system, while maintaining signal integrity over all of the channels, the optical transmitter should also be substantially linear and free of distortion.
To increase capacity over a fiber communication link, multi-wavelength transmission using wavelength division multiplexing (WDM) techniques has become relatively straightforward for transmission of digital data. For instance, multiplexing circuits can perform passive WDM and dense WDM (DWDM) multiplexing of multiple optical signals, such as to enable transmission of well over eighty optical signals at selected wavelengths in the 1400-1600 nm window. However, for certain industries, including but not limited to cable television (CATV) distribution systems that send bandwidth intensive media (e.g., audio and video information) utilizing highly bandwidth efficient, complex modulation (e.g., Quadrature Amplitude Modulation (QAM)), traditional baseband digital (e.g., On-Off-Keying (OOK)) transmission via WDM is considered inadequate due to the very inefficient bandwidth use associated with such transmission's operation. That is, traditional baseband digital schemes are no where near as bandwidth efficient as QAM and other forms of complex modulation, so that they effectively limit the useable bandwidth of a given fiber.
Initially, in the CATV field the DWDM wavelengths carried just eight RF quadrature amplitude modulation (QAM) narrowcast channels, containing a mix of video, high-speed data and voice, to each node, overlaid (that is linearly added in a RF channel-by-channel sense) above those frequencies already occupied by a tier of (typically 80) RF analog video carriers. While DWDM is more widely deployed, it has primarily been a bandwidth expansion tool for cable operators that had standardized on 1550 nm hybrid fiber/coax (HFC) networks. However, such systems tend to be limited by distortion and crosstalk, which can significantly degrade the carrier-to-noise ratio (CNR), which is a useful metric ultimately determining the Bit Error Ratio (BER) of the QAM data streams delivered.
Another approach for the cable television (CATV) distribution fiber plant employs DWDM transmission in the O-band via a standard single-mode fiber. Wavelength-division multiplexing allows logical segmentation of nodes while the 1310-nm wavelength allows the use of directly modulated diode lasers as transmitters, with the attendant lower complexity and lower cost. Because the chromatic dispersion is low, especially near 1310 nm, significant conversion of phase (frequency) modulation to amplitude modulation, which is a source of composite-second order (CSO), composite triple-beat (CTB), and cross-modulation distortion (XMD), is not the major contributor to nonlinear distortions. Despite the advantages associated with direct modulation in the O-band, four-wave mixing (FWM) is phase-matched or nearly phase-matched often resulting in large FWM products, such as crosstalk and other distortion, being generated. Raman coupling is also responsible for a dramatic increase in the noise coupled the analog modulation on each wavelength, greatly reducing the Carrier to Noise ratio (CNR). Thus, FWM must be accommodated to achieve desirable CNR in such a DWDM system. Raman coupling must be carefully avoided also to prevent debilitating loss of CNR.
Because of the inherent complexity and cost associated with implementing multi-channel WDM systems for optical transmission of complex modulated signals, such as for CATV distribution, there is further need for an approach that can effectively implement sparing for a failed transmitter.